Warn in ObjC++ when an 'auto' variable deduces type 'id'.

This could happen for cases like this:

- (NSArray *)getAllNames:(NSArray *)images {
  NSMutableArray *results = [NSMutableArray array];
  for (auto img in images) {
    [results addObject:img.name];
  }
  return results;
}

Here the property access will fail because 'img' has type 'id', rather than,
say, NSImage.

This warning will not fire in templated code, since the 'id' could have
come from a template parameter.

llvm-svn: 158239